Amplifier overload indicator



p'l 2l, 1936. A, R 4-IOHQKINS 2,038,110

AMPLIFIER OVERLOAD INDICATOR Filed July l5, 1932 INVENTOR ALVA R. HOPKINS ATTOR EY Patented Apr. 21, 1936 UNITED STATES PATENT OFFICE 2,038,110 AMPLIFIER OVERLOAD INDICATOR Alva R. Hopkins, Merchantville,

Radio Corporation of America,

Delaware Application July 15, 1932, Serial No. 622,617

My present invention relates to cators, and more particularly to a 4 Claims.

overload indidevice for nfull modulation.

Another important object of the present invention is to provide a glow discharge tube indicator for a power amplier stage of a radio receiver, the function of the glow tube being to inform the user of the receiver when the output stage is overloaded and causing distortion.

Another object of the invention is to provide in combination with the manual volume control device of a radio receiver and a push-pull power amplifier stage, a neon tube overload indicator adapted to be connected in the circuit of pull stage whereby the neon tube the pushis capable of serving as an indicator to inform the operator of the receiver when the manual volume control device should be adjusted for diminishing the input to the push-pull stage to el loading of the said push-pull stage,

iminate overit being noted that the average operator is unable to detect the level at which distortion begins, and, this device enables him to operate the amplifier at maximum eiiiciency and at the same time insure against distortion due to overload.

Still other objects of the present to improve generally the efficiency invention are of power amplier stages and to particularly provide an overload indicator for a power amplifier stage of a centralized radio system whichis not only durable and reliable in operation, but economically manufactured and installed.

The novel features which I believe to be characteristic of my invention are set ticularity in the appended claims, itself, however, as toboth its org forth in parthe invention anization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one cir cuit arrangement whereby my invention may be carried into effect.

ventional single arrangement for a centralized radio system, it will be observed that the channel includes a superheterodyne type of divided into two main portions,

radio receiver One of these N. J., assigner to a corporation of (Cl. Z50-20) main portions comprises the radio receiver proper, and includes the usual grounded antenna circuit A, G connected toa tuned radio frequency amplifier, the latter being connected in well known manner to a `first detector, a local oscillator also 5V being connected to the first detector. The first detector is followed by the usual intermediate frequency amplifier, second detector, and a stage of pushpull audio frequency amplification. In

a centralized radio system it is customary to provide this main receiver portion as a single unit on a panel of a channel rack.

The second portion of such an assembly usually comprises a power amplifier stage I and the conventional power supply circuit 2; the latter 15 usually includes the customary rectifier tubes adapted for connection to an alternating current supply. The power amplifier and power supply circuit are customarily provided as a unit independent of the aforementioned receiver unit, and is mounted on a panel of a channel rack immediately below the receiver panel. This channel rack construction is disclosed in application Serial No. 491,918 led on October 29, 1930, by Arthur F. Van Dyck. Since this channel rack construction does not comprise the subject matter of the present invention, it is believed sufficient for the purposes of this application to merely refer to the Van Dyck application.

Additionally, it is not believed necessary to go into the details of the superheterodyne receiver portion of the circuit since those skilled in the art are well acquainted with such a radio receiver which covers-the entire broadcast band from 550 to 1500 kilocycles. It is merely necessary to point out that the receiver may contain one stage of tuned radio frequency amplification ahead of the-first detector, and one stage of 175 kilocycle intermediate frequency amplification, and that the push-pull audio frequency amplifier may include 247 type tubes which are transformer coupled, as at M1, to the grids of the UX250s of the power amplifier stage I.

The manually operated volume control, conventionally represented by the arrow 3, is connected kbetween the output of the intermediate frequency amplifier and the grid of the second detector. 'I'his is a well known type of volume control device, and it is not believed necessary to show the connection of the variable resistor between the grid of the second detector and the secondary coil of the output transformer of the intermedi-ate frequency ampliiier. It is sufficient for the purposes of this application to point out that the numeral 3 designates a manual 55 volume control device which controls the delivery of signal energy to the second detector, and amplified signal energy to the input of the power amplifier stage I.

The power amplifier stage l comprises a pushpull stage using UX250 type tubes 4, 5. The power supply circuit 2 supplies the filtered rectified alternating current potential to the plates of 4 and 5 through a lead 6, and lead 'I supplies the heating current for the cathodes of tubes 4 and 5 from the power supply circuit in the usual manner. The input transformer M1 of the power amplifier stage I has a mid-tapped secondary, the usual balancing resistors 8 being connected in shunt with each half of the said secondary. The secondary coil of the output transformer M2 is arranged for connection to the usual monitoring speaker of the channel rack, and also to the distribution channel of the centralized radio system.

As is well known, the second-ary of the transformer M2 may be tapped for four normal output levels, the mid-tap being a common midpoint of all levels and grounded to provide for a balanced-to-ground-distribution system, thus decreasing cross-talk between different channels. The details of the wiring of the different output levels which are connected to the amplifier terminal board for distribution are not shown in order to preserve simplicity of description.

Across the grid circuit of one of the 250 tubes, for example tube 4, is connected the neon o-verload indicator Il), the indicator tube I0 being mounted on the front of the panel rack, as conventionally designated by the numeral Il, and is visible to the operator of the centralized radio system. The reason for placing the indicator on the output stage is that it is generally good practice to design the equipment so that the output stage is the first to overload. Therefore, this indicates the first distortion. This neon tube, or lamp, is provided in the front panel, and on the same panel which carries the power amplifier unit, to indicate any overload of the power amplifier stage I. The neon tube may also be said to be connected in the common input circuit of tubes 4 and 5 to indicate full modulation. Of course, as many channel racks as is desired, each including the apparatus shown on the drawing. may be provided in a single installation, and in that ca-se there would be a neon overload indicator on each channel rack.

The neon overload indicator is designed to flash when volts is applied to it, and the grid bias of the tubes 4, 5, is 84 volts. This bias voltage may be'obtained in any known manner such as by the connection of a C battery 20 as shown. If the volume control 3 of the receiver is set so that the neon tube is lit, it indicates that the output stage l is overloaded and causing distortion, as the tubes are drawing grid current. It is intended that in operation the volume control 3 of the receiver will be set so that the neon tube does not light. Then, as it flashes it serves as an indication that the operator should cut down the volume of the receiver. The neon lamp is lighted intermittently as the audio input to the stage l is excessive, the receiver output current being adjusted to a value or level just below the level where the lamp flashes at the peak values of the audio currents. Where the lamp flashes on the peaks the amplifier is overloaded.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, with a power amplifier stage including a source of modulated signal energy to be amplified, and a utilization output circuit, means connected to the input circuit of said ampliner and arranged to visually indicate only when the amplifier is drawing grid current due to overloading by the modulating signal energy, said stage including a pair of tubes connected in push-pull, and said means including a gas discharge device connected across the grid circuit of only one of the tubes.

2. In combination with a radio receiver including a tuned radio frequency amplifier, a first audio stage and a power amplifier stage including at least one vacuum tube having a cathode, control grid and audio output circuit, a volume control device located between the tuned radio frequency amplifier and the first audio stage, the grid of the power amplifier stage normally having a large negative bias with respect to its cathode, and a glow discharge tube designed to be inoperative at a voltage equal to the normal bias of said grid, one electrode of said glow tube being directly connected to said grid and the other electrode thereof being .directly connected to a point of fixed potential of said power stage grid circuit whereby said discharge tube glows only when the volume control device is adjusted to such a value as to cause grid current to flow in said grid circuit and a resistor connected in shunt across the electrodes of said glow tube.

3. A signaling system including a superheterodyne receiver having a manual volume control device connected between the intermediate frequency amplifier and second detector, a pushpull power amplifier stage connected to the output of said receiver, and a visual overload indicator connected in the input circuit of said pushpull stage, said indicato-r being designed to be inoperative at a predetermined maximum setting of said volume control.

4. In an amplifying arrangement, the combination of an audio frequency amplifier, means for impressing modulated signal currents across the input terminals of said amplifier, a power amplifier comprising two tubes connected in pushpull, a transformer having its primary connected to the output of said first named amplifier and the opposite ends of its secondary to the grids of said power amplifier stage, the said grids having normally a large negative bias, a balancing resistor connected between each of said grids and the midpoint of said secondary coil and a. glow Idischarge tube having its electrodes connected to the opposite ends of one of said balancing resistors, said discharge tube being designed to have a breakdown voltage in excess of the normal bias voltage of said grids.

ALVA R. HOPKINS. 

